Believing in aliens means having the idea that biological life can be found somewhere on another galaxy/solar system. A god would be an omnipotent being that created the universe and might not be biological or even physical. It is an entirely different thing.
The existence of aliens is a testable hypothesis and a phenomenon that can be predicted with the knowledge we have. The existence of god is not testable and requires an assumption leading (in many cases) to contradictions.
I sometimes like to think of how giant the observable universe is. we have no idea how much bigger it can be. The more matter there is in the universe, the closer it becomes for small possibilities such as another intelligent life to become a reality.
This assumes either that belief in the existence of aliens is irrational, and therefore belief in the existence of a God is also irrational, or that belief in aliens is completely rational, and therefore belief in the existence of a God is also rational.